Each manufacturer has its own way to make a tent waterproof, UV resistant and at the same time breathable. The waterproofness of the material is rated in mm and this is a specification you want to look for when you are comparing tents. Numbers go between 800 mm and 10,000 mm.

The number refers to a water column height in millimeters. For example, 1500 mm means that the fabric will withstand a 1500mm (5') column of water for more than one minute before a single drop might appear through the fabric. That's strong enough to prevent rain from leaking in a 75 mph hurricane-force storm. In the outdoor industry, they use this number as shorthand to describe the amount of coating used to make the nylon or polyester tents waterproof. Some people may also refer to this value as the hydrostatic head pressure.

The general consensus is that from 800-1000mm on, the material is waterproof. BUT, it is better to have a high water column because coatings will wear off, and the waterproofness value will decrease.

The higher the number is, the longer the waterproof ability of the material is going to last, and the more durable the tent is going to be; but the weight of the material - therefore the tent - will be noticeably higher. Long distance bicycle travelers usually choose tent with a high waterproof rating.

Did you know? Several tents from the UK manufacturer Terra Nova are among the highest rated tents in terms of waterproof ability.

The PU (polyurethane) coating is one of the strongest and most abrasion-resistant kinds of coating, but manufacturers usually don't tell what coating they use so you'll need to contact them to find out. PU optimizes the waterproofness and strength while still being light-weight. Manufacturers can maximize the amount of coating only to a certain point. More coating means more weight and more cost. Also chemicals in coating weaken the fibers, so at some point, increasing the water column value too much will decrease the fabric strength. This is why 10000 is the highest value you can find.

From my personal experience a 7000 mm Ripstop fly, which was used in rough condition, with poor maintenance (I didn't know better) started to leak after about 500 nights of use. There are no test standards or easy ways to verify the water column value for products on the market so you have to trust the manufacturer.